1. Field of the Invention
This invention relates to subscriber loop carrier telepone systems and, more particularly, to electronic ringing generators for providing ringing signals in such systems.
2. Description of the Prior Art
In carrier-derived telephone circuits it is necessary to provide both a means of signaling the need for ringing signals to a remote subscriber and also to provide a source of ringing signals to be applied to the subscriber's line. In single channel carrier systems such as that disclosed in L. Krasin U.S. Pat. No. 3,471,650, granted Oct. 7, 1969, the need for ringing signals is transmitted by applying the carrier signal to the line and interrupting it at a 20 Hz rate. The signal can then be used at the remote terminal to both indicate the need for ringing and as a control signal to aid in the generation of the ringing signal.
Several problems arise in remotely located telephone ringing signal generators. It is difficult, for example, to design oscillators and amplifiers to operate at such a low frequency rate and deliver the required high ringing voltage. Moreover, ringing power requirements tend to be high in order to operate the mechanical portions of a telephone ringer. The power required for these signals constitues a considerable load on a remote battery and, moreover, must be carefully isolated from the talking circuits when ringing is not ocurring to prevent undue loading on the talking path. It is also necessary to detect ring tripping, i.e., the need to terminate ringing when the subscriber goes off-hook.
In many ringing signal generators, a direct-current converter circuit is utilized to generate a high DC ringing voltage from a low voltage battery. Ringing signals are generated from this high DC voltage by interrupting the voltage at a 20 Hz rate and interrupting the 20 Hz signal during the silent intervals of the ringing cycle. Many problems arise in attempts to interrupt this high voltage.